1. Field of the Invention
The present invention relates to a mobile communication system and, in particular, to an apparatus and method for compensating a received signal level transmitted from a mobile terminal via a measurement report message.
2. Description of the Related Art
Typically, noise and interference are more significant problems in wireless communication systems than in hard-wired communication systems. Thus, various modulation, amplification, mixing, and filtering techniques have been employed for improving communication reliability in wireless systems.
In order to process wireless signals, a radio frequency (RF) module is employed in each wireless communication device. The RF unit is responsible for converting a baseband signal into a radio signal and amplifying the radio signal to be robust to interferences at its transmission side. The RF unit is also responsible for filtering, low-noise amplifying, and down-converting a desired signal to recover the transmitted signal.
Now days, mobile communication systems have been developed to support a packet data service in addition to the basic voice communication service. With the advance of the mobile communication system, the management of the system has become complex.
In order to efficiently manage the communications between a base station (BS) and mobile stations (MSs), each MS collects and reports channel status to the BS in the format of a measurement report.
The measurement report includes information such as a serving cell reception level (RXLEV-FULL(SUB)-SERVING-CELL), a serving cell reception quality (RXQUAL-FULL(SUB)-SERVING-CELL), a neighbor cell broadcast channel (BCCH-FREQ-NCELL), and a neighbor cell list (BASIC-NCELL, BASIC-NCELL, . . . ).
Accordingly, the BS checks the reception level (RXLEV) and reception quality (RXQUAL) at the mobile station and guides the MS having a low RXLEV and RXQUAL to perform a handover to an optimal BS with reference to the information on neighbor cells.
In the meantime, when more than two MSs are associated with a BS at an adjacent location at the same, transmission signals of the neighbor MSs can act as interferences in a receive (RX) band of a specific MS.
Typically, a transmission/reception (TX-RX) isolation in a front end module and RF transceiver of an MS is not perfect, such that the transmission power of a neighbor MS may be received. In this case, the neighbor transmission power leaks in the RX band of the neighbor MS, thus resulting in influence to a direct current (DC) signal of In Phase (I-Phase) signal components and Quadrature phase (Q-Phase) signal components (IQ).
In a case where a TX IQ timeslot of an interferer MS is overlapped with an RX IQ timeslot of the MS, the IQ signal can be influenced by the TX power, thereby resulting in an increase in DC offset of the RX IQ signals. This causes a reduction in reception quality of the RX signal.
The above effect occurs in a Direct Conversion Receiver (DCR) which does not use an intermediate frequency (IF). However, this interference may occur in Near-Zero Intermediate Frequency (NZIF) transceiver.
FIG. 1 is a block diagram illustrating a configuration of a conventional DCR.
Referring to FIG. 1, the DCR includes a low-noise amplifier (LNA) 10, a filter 20, a local oscillator 30, and a mixer 40.
In an MS employing the above-configured conventional DCR, if a TX signal of a neighbor MS acts as interference to RX signal of the MS, a current leakage occurs on a path of the local oscillator 30 such that signals input through a path 1 and a path 2 are mixed by the mixer 40. The mixture of the signals input through the paths 1 and 2 causes a DC offset. Such a problem is derived from a characteristic of the DCR of which local oscillator frequency should be equal to the received signal frequency.
Since the received signal level increases by the interference, the neighbor MS's TX signal and thus the measurement value reported to the network is considered good enough. As a result, the network determines that the channel between the MS and BS is good, thereby not transmitting a handover command although the channel state is actually bad. Particularly, the possibility of the TX spurious in RX band significantly increases in a weak transmit power area. In this case, since the transmit power of neighbor cell is also weak at the location of the MS, a serving cell maintains the operation with the MS, thus resulting in call drop.